Saving a Colonial Masterpiece: Mount Pleasant Restored

Mount Pleasant survives as one of the greatest American houses of the eighteenth century, a testament to the skill and ambition of colonial builders and clients. Remarkably, the estate remains largely intact on its original site as an ensemble of buildings within a landscape that offers the parklike view and river prospect first enjoyed by its original owners.

Like any modern house, Mount Pleasant needed a new roof to replace one put on thirty years ago. Unlike a modern house, Mount Pleasant is 240 years old and a National Historic Landmark administered by the Philadelphia Museum of Art on behalf of the City of Philadelphia. Therefore, before any work could begin, a preservation team of architects, conservators, engineers, and carpenters conducted a structural survey of the main house and its two pavilions. This 2003 inspection revealed serious deterioration beneath the shingles that extended to the timber frame supporting the main building's roof. This damage threatened the integrity of the entire structure.

In making a plan to address these problems, the team relied on principles that guide all conservation of works of art in museum collections:

Preservation of Original Material. The eighteenth-century timber frame represents the materials and practices of the home's original craftspeople, whose engineering and design created a building able to stand two and a half centuries.
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Restoration of Original Appearance. Where new material or construction was necessary, the team undertook extensive study to ensure that it was historically accurate.
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Preserving Original Material

The Roof Structure

Although the shingles on Mount Pleasant's roof have been replaced many times since it was first completed in 1765, the roof's timber frame is original. Unfortunately, sections of critical timber supports had rotted over time and no longer transferred the weight of the roof onto the exterior stone walls. Instead, the massive weight rested on interior walls, which were not designed to carry such a load.

The challenge the preservation team faced was how to restore the requisite strength to the roof structure while preserving as much of the original wood as possible.

Queen-Post Truss System

Mount Pleasant's timber frame utilizes a system of four queen-post trusses, each running from the front of the house to the back. Each truss consists of a top chord, a bottom chord (or tie beam), two queen posts, two diagonal braces, and two principal rafters. These elements transfer the weight of the roof and the third floor to the front and back load-bearing walls, which are made of stone.

The shaded areas on the drawing above show where rot frequently occurred on the trusses. Seven of the eight truss ends needed repair.

The Process

Preparation

Before restoration could begin, carpenters encased Mount Pleasant in scaffolding and placed a temporary plastic covering over the top of the building. Plywood protected the windows and stairs. To support the weight of the third floor and the roof frame while work on the truss ends proceeded, they installed a temporary grid of steel beams. Once carpenters replaced the decayed sections of the timber frame, the steel could be removed.

Assessment of Damage

This purlin (a horizontal structural timber) demonstrates the degree of rot found in a number of places in the frame of the roof. The majority of rot resulted from water leaking through the roof over the past 240 years; fungus and vermin also caused damage. Underneath the dormer window, another purlin had completely rotted. Only its two ends remained. Without the purlin to support it, the dormer window sill pitched inwards, allowing water to leak into the interior of the house.

Preservation and Restoration of the Timber Frame

This photograph shows three repaired truss ends. In order to restore the structural integrity of the truss, new wood was joined to eighteenth-century wood that was still sound. To do this, carpenters inserted a steel plate into the end of each member, using epoxy resin to hold everything in place. Some original truss elements needed little or no repair, like the iron strap that holds the joint together and the diagonal brace. Both are visible in the truss in the foreground.

Restoring Original Appearance

The Dormers

As part of the roof, the dormers also needed repair and restoration. Some elements of the dormers were modern, raising questions about their original appearance. With no drawings or specific descriptions of how they looked in 1765, the preservation team relied on clues uncovered in the removal of the modern materials. This physical evidence, as well as design sources—books and similar houses of the period—helped determine the most historically accurate restoration and answered the following three questions.

Using Physical Evidence and Design Sources

Dormer Scroll Brackets

Question:

What did Mount Pleasant's original scroll brackets look like? When architects and conservators inspected them for the recent restoration, they found that the brackets and the sills on which they sit had been restored in 1975. These brackets were probably the third set to decorate the house since 1765.

Physical Evidence:
Shadow marks in the paint on the dormer frame indicated the height and the thickness of the original brackets. The sill into which the brackets fit was modern, so similar shadow marks showing the width of the brackets were not present. Conservators discovered the precise length of the original sill by measuring the chamfered (beveled) edge cut for it on the purlin. With that measurement, they figured out the width of the original bracket that sat on top of the sill.

Design Sources:

The Carpenters' Company 1786 Rule Book and study of Cliveden, a 1760s house in Germantown, helped determine a historically accurate design for Mount Pleasant's scroll brackets.

This page from The Carpenters' Company 1786 Rule Book shows early American dormer designs. (Courtesy of The Carpenters' Company of the City and County of Philadelphia)

Cliveden, the home of Benjamin Chew in Germantown (Philadelphia), was completed in 1767 and is currently owned by the National Trust for Historic Preservation. Mount Pleasant and Cliveden are from the same period and share many architectural features. Conservators studied Cliveden's scroll brackets, eighteenth-century originals, as models for Mount Pleasant's restoration.
Photo by John Chew. Courtesy of Cliveden, a National Trust Historic Site.

Conclusion:

Conservators integrated physical evidence and eighteenth-century design sources with their knowledge of Mount Pleasant's architectural and ornamental style into a final design for the new scroll brackets.

Decorative Molding

Before restoration, dormers with
decorative molding

Question:

Was the decorative molding outlining the arched windows an original 1760s design feature?

Physical Evidence:
The decorative molding seen on Mount Pleasant before the restoration does not appear in any photographs of the house prior to 1926, suggesting that this particular molding was not original. After the preservation team removed the modern molding, no nail holes from earlier ones were found.

Under the modern molding, the team discovered many layers of paint across the entire face of the dormers, which confirmed that no molding had been attached prior to 1926.

Conclusion:
The molding was a 1926 embellishment to the dormers. As there was no evidence of prior molding, the preservation team did not replace it in the restoration.

Dormer Cheeks

Before restoration, dormer
cheeks with shingles

Question:

What treatment did the cheeks (sides) of Mount Pleasant's original dormers have? Two possible treatments—boards or shingles—are historically accurate to the eighteenth century.

Physical Evidence:
After the removal of the modern shingles from the dormer cheeks, the preservation team observed nail holes arranged in horizontal rows spaced approximately 9 inches apart.

In addition, five 9 1/4-inch boards fit exactly into the space of each dormer cheek. Conservators discovered similar eighteenth-century boards in other parts of Mount Pleasant's main building and pavilions, indicating the type the colonial builders used.

Finally, the preservation team observed that the cornice and the face of the dormers were cut to receive a board 9 1/4 inches wide, not shingles.

Conclusion:
Boards, not shingles, originally covered the dormer cheeks. Carpenters placed five boards, each 9 1/4 inches wide, in horizontal rows, decorating the lower edge of each with a bead to match boards found in all of Mount Pleasant's buildings.